Since the mid 1930s, vehicles such as automobiles and light trucks have predominantly utilized hydraulic brake systems having a pedal-operated master cylinder supplying pressurized hydraulic fluid to disk or drum braking devices at each wheel. Early hydraulic brake systems utilized a single hydraulic fluid circuit supplying pressurized fluid from the master cylinder to all four wheels of the vehicle. A break in the fluid circuit anywhere rendered the entire hydraulic brake system inoperative.
In order to prevent a total loss of vehicle hydraulic braking in the event of a failure of part of the system, failsafe hydraulic split brake systems were developed that provided two separate fluid circuits from the master cylinder, configured such that a failure of either of the two fluid circuits would still leave hydraulic brakes operative on at least two wheels of the vehicle. In rear wheel drive automobiles and light trucks, one fluid circuit typically served the front wheels and the other fluid circuit served the rear wheels to provide a front/rear (F/R) failsafe hydraulic split system. Front wheel drive vehicles typically used a diagonal failsafe hydraulic split system, having one front wheel and the diagonally opposite rear wheel of the vehicle on one fluid circuit, and the other front wheel and its diagonally opposite rear wheel on the second fluid circuit. Government stopping distance regulations were passed for failed brake system performance that required brake systems to be configured such that a single failure of the braking system would still leave the brakes on at least two wheels of the vehicle operational.
In the years since hydraulic brake systems became the norm, many additional features have been added to further enhance safe operation and optimize vehicle performance. Modern brake systems often include a booster that amplifies force exerted on the brake pedal to provide power brakes that allow actuation with significantly less force applied to the brake pedal than required for a non-boosted brake system. Anti-lock brake systems (ABS) were developed in which valves controlling fluid flow to each wheel of the vehicle were pulsed, in response to signals received from rotation sensors monitoring each wheel, to preclude locking the brakes on slippery road surfaces. Traction control systems (TCS) were added that controlled both the brakes and the engine throttle setting to improve traction and handling of the vehicle during maneuvers, such as acceleration or turning, when the brakes are not being applied by the operator. Vehicle dynamics control (VDC) further advanced the level of sophistication of brake systems to utilize a number of sensors throughout the vehicle, and a more advanced onboard computer with higher throughput to monitor forces acting on the vehicle, together with inputs indicating operational commands from the operator applied to the steering, braking, and drive systems. VDC analyzes the data received from the sensors and coordinates operation of the various elements of the vehicle brake system, power-train, and, in some cases, suspension to provide enhanced vehicle safety or performance of the vehicle.
The addition of these enhancements has made hydraulic brake systems very complex. Numerous valves, sensors, and electronic control components are required. Brake systems offering one or more types of automated control operating modes, such as ABS, TCS and VDC, are known as “controlled braking systems.”